The present invention relates most generally to the semiconductor manufacturing industry and more particularly to the polishing pads used in chemical mechanical polishing tools used in the semiconductor manufacturing industry.
Chemical mechanical polishing (CMP) operations are widely used in the semiconductor manufacturing industry for polishing film structures during the fabrication of semiconductor devices. A chemical mechanical polishing operation is generally used for several operations during the fabrication of each semiconductor device. Consumable costs associated with CMP operations represent an ever-increasing portion of total production costs associated with the semiconductor manufacturing industry. An example of such a consumable item is the polishing pads used in CMP tools for polishing semiconductor substrates.
A chemical mechanical polishing process may be accomplished by an abrasive slurry lapping process in which a semiconductor wafer mounted on a rotating carrier is brought into contact with a rotating polishing pad upon which is introduced a slurry of insoluble abrasive particles suspended in a liquid. The slurry may additionally be acidic or basic in nature. As such, CMP is accomplished using both mechanical abrasion and chemical action. Material is removed from the semiconductor wafer surface due to both the mechanical buffing action and the chemical action of the acid or base.
Various CMP tools are known in the art. A typical CMP tool includes a rotatable circular polishing platen having a circular polishing pad mounted thereon. A rotatable polishing head or carrier adapted for holding and often rotating a substrate such as a semiconductor wafer is suspended over the platen. The carrier and platen are rotated by separate motors. The slurry is introduced onto the polishing pad surface. The semiconductor wafer held by the carrier is brought into contact with the pad and is polished due to the mechanically abrasive action of the abrasive particles and the chemical action of the slurry. The polishing pad includes an upper portion typically formed of a urethane material consisting of, for example, a flexible non-woven fabric impregnated with foamed urethane. Such a urethane pad has a plurality of fine voids at the pad surface. The voids typically extend perpendicularly away from the polishing pad surface and create pores at the pad surface. The voids typically extend perpendicularly through the upper portion of the polishing pad. The slurry is received and retained in these pores, enabling the pad to chemically and mechanically polish the semiconductor wafer. The polishing pad also includes a lower portion formed of a spongy, resilient material.
During the CMP process used to polish the wafer surface, upright sharp points on the surface of the pad may be worn, compressed, or depressed by the pressure applied from the wafer to the pad and any motion imparted upon the wafer. The voids of the pad may also be plugged with the mixture of slurry and solid wafer material separated from the wafer surface due to the wear of the surface as a result of the polishing. In this manner, a glazing phenomenon occurs on the pad surface.
When the voids on the surface of the pad become plugged, it is difficult for the pad to hold the slurry. The degree of pad pore saturation is reduced. This, along with the wearing of sharp points on the surface of the pad, degrades wafer polishing efficiency and repeatability as well as the uniformity of the polished wafer surface. In order to solve such a problem, the polishing pad surface is typically conditioned by being ground at the surface using a diamond coated disk after being used for several wafers or tens of wafers. The conditioning process removes a surface layer laminated upon the pad and counteracts the glazing phenomenon which occurs. That is, a fresh new pad surface is periodically formed by the conditioning process. The freshly formed pad surface formed by the conditioning process includes a desired and consistent degree of pad roughness, and includes open pores capable of receiving and retaining the slurry. As such, the conditioning process is periodically and regularly carried out.
The conditioning process, however, includes the following limitations. Diamond grains may separate from the diamond disk during the conditioning of the pad and form scratches on the surface of a wafer being polished. Additionally, the pad and wafers may be contaminated by metal grains separated from the disk on which the diamonds are disposed. Furthermore, the conditioning processes themselves can be time consuming and also result in a yield degradation.
Moreover, each time the pad is conditioned, a large amount of the pad material is removed. As a result, the lifetime of the consumable polishing pad is shortened due to multiple conditioning operations being carried out. The consumable polishing pad is a costly item. Additionally, when the consumable polishing pad is replaced, the CMP tool is unavailable for production use. The maintenance procedure used to replace the polishing pad may be time consuming and will often require that extensive and time-consuming warm-up procedures are conducted subsequent to pad replacement. As a result of the time required to replace the pad and to carry out the subsequent warm-up procedures required, the CMP tool is unavailable for an extended time. This results in a further yield degradation.
As such, it can be seen that what is desired in the art is a polishing pad which includes an extended lifetime, does not require frequent replacement, and maintains a consistent polishing surface having the same degree of roughness and including pores which retain the polishing slurry and therefore promote a consistent and uniform removal rate, a good planarizing ability, and a reliable, repeatable and efficient polishing process.
To meet these and other needs, and in view of its purposes, the present invention provides a polishing pad for use in a chemical mechanical polishing tool and a method for forming the same. The upper portion of the polishing pad is a closed-cell elastomer. The upper portion includes bubbles contained therewithin. Because of the random distribution of even sized bubbles within the elastomer material, conditioning is not required as new bubbles are continuously exposed as the pad wears out during polishing. The newly exposed bubbles at the polishing surface are capable of receiving and retaining the polishing slurry. The present invention also provides a method for forming the polishing pad by introducing gas bubbles into the fluid elastomer material which will form the polishing pad. Process conditions are chosen to maintain the bubbles within the elastomer material, as the elastomer material is formed into a solid cake from which individual polishing pads will be formed by slicing.